The development of cracking catalysts has heretofor generally been limited to the preparation of modified zeolites for use as cracking catalysts and to the interaction of such zeolites with other inorganic oxide materials. The following patents are representative of the prior art dealing with zeolite based cracking catalysts:
The use of conversion catalysts formed from a zeolite dispersed in a siliceous matrix has been disclosed in U.S. Pat. Nos. 3,140,249 and 3,352,796.
The use of blended matrix components, e.g., a catalyst comprising a zeolite, an inorganic oxide matrix and inert fines, which may be alpha alumina, is disclosed in U.S. Pat. No. 3,312,615. Catalysts comprising an amorphous silica-alumina, separately added alumina and a zeolite are disclosed in U.S. Pat. No. 3,542,670 and catalyst comprising a zeolite, an amorphous hydrous alumina and alumina monohydrate are disclosed in U.S. Pat. No. 3,428,550.
It has been disclosed that the steam and thermal stability of Y zeolites can be improved by the use of zeolites having a low level of alkali metal content and a unit cell size less than about 24.45 Angstroms (See: U.S. Pat. Nos. 3,293,192 and Re. 28,629 (Reissue of U.S. Pat. No. 3,402,996)).
Further, it has been disclosed (U.S. Pat. No. 3,591,488) that the hydrogen or ammonium form of a zeolite may be treated with H.sub.2 O at a temperature ranging from about 800.degree. to about 1500.degree. F., or then subsequently cation exchange the steam and water treated zeolite with cations which may be rare earth metal cations. The method increases the silica to alumina mole ratio of the zeolite and also the defect structure. U.S. Pat. No. 3,676,368 discloses a rare earth exchanged-hydrogen faujasite containing from 6 to 14 percent rare earth oxides. U.S. Pat. No. 3,957,623 discloses a rare earth exchanged zeolite having a total of 1 to 1 weight percent rare earth metal oxide. U.S. Pat. No. 3,607,043 discloses a process for preparing a zeolite having a rare earth content of 0.3 to 10 weight percent.
U.S. Pat. No. 4,036,739 discloses hydrothermally stable and ammonia stable Y zeolite in which a sodium Y zeolite is ion exchanged to partially exchange sodium ions for ammonium ions, followed by steam calcination and a further ion exchange with ammonium to reduce the final sodium oxide content to below 1 weight percent, followed by calcination of the reexchanged product, or according to U.S. Pat. No. 3,781,199, the second calcination may be conducted after the zeolite is admixed with a refractory oxide.
The above discussed prior art is representative of past and present day formulations of catalysts for fluid catalytic cracking (FCC). Recently a new class of materials was disclosed in U.S. Pat. No. 4,440,871. The materials of U.S. Pat. No. 4,440,871 are crystalline microporous silicoaluminophosphate molecular sieves and are disclosed as employable in cracking processes. Several of the "SAPOs" of U.S. Pat. No. 4,440,871 were evaluated for their catalytic cracking activity by use of an n-butane cracking test from which data a first-order rate constant was calculated. Although the first order rate constants for all the SAPOs tested showed such to have catalytic activity, the rate constants varied from 0.2 to 7.4. The use of a mixture of aluminosilicates and specific silicoaluminosilicates is disclosed in copending U.S. Ser. No. 675,283, filed concurrently herewith and commonly assigned.
The instant invention relates to cracking catalysts and to fluid catalytic cracking processes. The catalysts comprise a specific class of silicoaluminophosphate molecular sieves disclosed in U.S. Pat. No. 4,440,871 having particular pore sizes and are preferably employed with at least one inorganic oxide present as a binder and/or matrix component.